Such respiration systems known from the state of the art, for example, EP 0 894 506 B1, first have the advantage that the anesthetic gas is guided in a respiration circuit, so that a large part of the expensive anesthetic gas can be reused. Only CO2 has to be removed from the circulating gas flow by means of an absorber, and the oxygen concentration has to be kept at a preset level.
By means of the manual respiration bag which is likewise contained in the circuit, it is possible for an anesthesiologist to manually adjust the pressure operating during the inspiration and the duration of the inspiration.
However, the following problem arises in such systems. During expiration, resistance or pressure opposing the expiration should not exceed a certain value, whereby, especially at the end of the expiration cycle, the pressure in the expiration branch may rise, for example, when the capacity of the reservoir is not sufficient. So that this resistance does not become unacceptably high, a so-called anesthetic gas discharge valve (hereinafter “NGF valve”), which opens in case of an excess pressure in the expiration branch of 1.2±0.1 mbar, is provided in the expiration branch. In the state of the art, this valve is usually designed, such that a valve body is pressed against a valve seat by means of a spring, whereby the prestress of the spring determines the threshold, at which the valve opens.
The flow in the expiration branch, especially at the beginning of expiration, is not constant, and also the pressure, which builds up during expiration at the NGF valve, fluctuates and has brief peaks or large pressure gradients in the range of
                    Δ        ⁢                                  ⁢        p                    Δ        ⁢                                  ⁢        t              =                  5        ⁢                                  ⁢        mbar                    0.5        ⁢                                  ⁢        sec              ,so that pressures briefly lying above the above-mentioned threshold at the NGF valve build up, which consequently bring about the situation that the gas in the supply line to the manual respiration bag has an inertia. On top of that, the NGF valve also briefly opens, although a pressure above the threshold has not built up for a longer period in the expiration branch. Thus, a so-called “rattling” of the NGF valve occurs, which leads to an unnecessary loss of anesthetic gas.